Rectifier tube cathode and heater

[CraigNS]

I'm new to tubes but not to electronics. It looks like the common arrangement for rectifier tubes is having the cathode and one side of the heater to a common pin (8 on a 5Y3GT). What I'm struggling to understand is what keeps the rectified output voltage from feeding back into the transformer winding providing the heater voltage as it's much lower (5 or so vs 200 plus) Maybe a phase thing, but looks like half of the phase would be cancelled out.

Explanations or what am I missing?

Craig

[sparkydave]

Hi CraigNS, the 5Y3 doesn't have a cathode, the heater itself is the cathode. The 5 volt winding in the power transformer is isolated from the high voltage windings, so it floats at the rectified output voltage, but never "sees" the high voltage. Since the rectifier output picks off one side of the heater, there is a slight imbalance because you correctly point out that it bucks or boosts by a few volts. If you looked at the rectified output with no filter capacitors, you would see alternating phases where one phase might be a few volts higher, and 1/120th of a second later (if you're in the U.S.) it would be a few volts lower. The filter capacitors take care of this variation. An indirectly heated rectifier like the 5AR4 doesn't have this problem because it has a separate cathode.

[payloadde]

Originally Posted by CraigNS

... what keeps the rectified output voltage from feeding back into the transformer winding providing the heater voltage as it's much lower (5 or so vs 200 plus) Maybe a phase thing, but looks like half of the phase would be cancelled out ...

nothing, the heater winding is indeed floating at B+ potential
that's why the heater winding must be separate and not connected to anything else (except the heater and the cathode)
because it is only connected at one end to B+ no current can flow, open circuit ...

[CraigNS]

I see, so because the heater winding is completely separate from the B+ winding, it's isolated from the circuit. But it will still provide a negative 6 volt potential to the B+ circuit won't it?


So there is a bit of pumping affect, but it's lagged from the ac output.

[sparkydave]

No, it will add to the rectified B+ for half of an AC cycle and subtract for half a cycle; the filament supply is still AC. The average DC value over one line cycle is the same.

[Eli Duttman]

The 5 volt rectifiers come in 2 "flavors": directly heated and half indirectly heated.

A half indirectly heated rectifier, like the 5V4, has a cathode sleeve connected to pin 8, along with 1 end of the heater. That method of construction avoids dealing with heater to cathode potential limits. You take the B+ from pin 8 with tubes of this construction.

The filament is the cathode in a directly heated rectifier, like the 5U4. As a previous poster stated, there is a minor hum penalty when the B+ is taken from either end of the filament. Best results, from a hum perspective, are obtained by taking the B+ off a center tap on the 5 V. winding, when filamentary rectifier types are employed.

As people like to "roll" tubes, taking the B+ from pin 8 is the usual state of affairs.